Put a large pan of salted water on to boil. Scrub the potatoes and chop them into equal-sized chunks. Set them aside. Peel and finely chop the onion.
Rub 1 tsp oil into each lamb chop and season them with salt and pepper. Set a frying pan over a high heat and warm it for 2 mins. Add the lamb chops to the pan. Fry for 4 mins on each side, so they're browned. Lift the lamb chops out of the pan and pop on a plate.
Turn the heat down under the pan. Add the onion. Season with a pinch of salt and pepper. Gently cook for 5 mins till the onion is golden, stirring now and then.
While the onion fries, halve the pepper. Scoop out the seeds and white bits. Roughly chop it. Halve the tomatoes. Peel and crush the garlic clove.
Stir the garlic into the onion. Cook and stir for 30 secs. Add the pepper, tomatoes and most of the thyme sprigs. Cook, stirring occasionally, for 10 mins till the tomatoes have broken down.
The pan of water should be boiling by now. Add the potatoes. Simmer for 15 mins, till the potatoes are tender when pressed with a fork.
When the tomatoes and peppers have cooked for 10 mins, add the lamb chops back into the pan. Pop on a lid and cook for 10 mins. Add 1 tsp of the butter to the pan and let it melt into the sauce. Taste the sauce and adjust the seasoning.
Drain the potatoes. Tip them back into the pan. Add the rest of the butter and toss to mix. Serve the lamb chops with the buttered potatoes, garnished with the remaining thyme leaves.